Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
CURRENT ASSETS :
Cash and cash equivalents	$ 374	$ 477	$ 291	$ 272
Short-term investments	394	523
Receivables, net	2,815	2,896
Fiduciary assets	11,638	11,871
Intercompany receivables	0	0
Other current assets	602	563
Total Current Assets	15,823	16,330
Goodwill	8,860	8,997	8,943
Intangible assets, net	2,520	2,578
Fixed assets, net	765	791
Investments	143	132
Non-current deferred tax assets	144	193
Intercompany receivables	0	0
Other non-current assets	1,517	1,230
Investment in subsidiary	0	0
TOTAL ASSETS	29,772	30,251
CURRENT LIABILITIES
Fiduciary liabilities	11,638	11,871
Short-term debt and current portion of long-term debt	783	703
Accounts payable and accrued liabilities	1,805	1,931
Intercompany payables	0	0
Other current liabilities	788	906
Total Current Liabilities	15,014	15,411
Long-term debt	4,799	3,686
Non-current deferred tax liabilities	313	420
Pension, other post-retirement and other post-employment liabilities	2,141	1,607
Intercompany payables	0	0
Other non-current liabilities	874	932
TOTAL LIABILITIES	23,141	22,056
TOTAL AON SHAREHOLDERS’ EQUITY	6,571	8,145
Noncontrolling interests	60	50
TOTAL EQUITY	6,631	8,195	7,805	8,120
TOTAL LIABILITIES AND EQUITY	29,772	30,251
Aon plc [Member]
CURRENT ASSETS :
Cash and cash equivalents	0	0	131	0
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	455	186
Other current assets	2	0
Total Current Assets	457	186
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	0	0
Non-current deferred tax assets	159	140
Intercompany receivables	7,399	7,166
Other non-current assets	20	6
Investment in subsidiary	4,962	4,607
TOTAL ASSETS	12,997	12,105
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	0	0
Accounts payable and accrued liabilities	3,755	1,036
Intercompany payables	122	15
Other current liabilities	0	12
Total Current Liabilities	3,877	1,063
Long-term debt	2,544	792
Non-current deferred tax liabilities	0	0
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	0	2,100
Other non-current liabilities	5	5
TOTAL LIABILITIES	6,426	3,960
TOTAL AON SHAREHOLDERS’ EQUITY	6,571	8,145
Noncontrolling interests	0	0
TOTAL EQUITY	6,571	8,145
TOTAL LIABILITIES AND EQUITY	12,997	12,105
Aon Corporation [Member]
CURRENT ASSETS :
Cash and cash equivalents	2,727	247	199	0
Short-term investments	165	163
Receivables, net	0	4
Fiduciary assets	0	0
Intercompany receivables	2,814	3,503
Other current assets	226	69
Total Current Assets	5,932	3,986
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	81	57
Non-current deferred tax assets	570	465
Intercompany receivables	600	2,178
Other non-current assets	46	95
Investment in subsidiary	15,200	11,694
TOTAL ASSETS	22,429	18,475
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	767	0
Accounts payable and accrued liabilities	58	62
Intercompany payables	8,960	5,449
Other current liabilities	49	47
Total Current Liabilities	9,834	5,558
Long-term debt	1,917	2,512
Non-current deferred tax liabilities	0	0
Pension, other post-retirement and other post-employment liabilities	1,396	925
Intercompany payables	7,277	7,267
Other non-current liabilities	125	159
TOTAL LIABILITIES	20,549	16,421
TOTAL AON SHAREHOLDERS’ EQUITY	1,880	2,054
Noncontrolling interests	0	0
TOTAL EQUITY	1,880	2,054
TOTAL LIABILITIES AND EQUITY	22,429	18,475
Other Non-Guarantor Subsidiaries [Member]
CURRENT ASSETS :
Cash and cash equivalents	1,361	1,246	0	293
Short-term investments	229	360
Receivables, net	2,815	2,892
Fiduciary assets	11,638	11,871
Intercompany receivables	9,156	5,452
Other current assets	407	513
Total Current Assets	25,606	22,334
Goodwill	8,860	8,997
Intangible assets, net	2,520	2,578
Fixed assets, net	765	791
Investments	62	75
Non-current deferred tax assets	113	193
Intercompany receivables	111	2,201
Other non-current assets	1,543	1,228
Investment in subsidiary	0	0
TOTAL ASSETS	39,580	38,397
CURRENT LIABILITIES
Fiduciary liabilities	11,638	11,871
Short-term debt and current portion of long-term debt	16	707
Accounts payable and accrued liabilities	1,706	1,849
Intercompany payables	3,343	3,677
Other current liabilities	772	866
Total Current Liabilities	17,475	18,970
Long-term debt	338	378
Non-current deferred tax liabilities	1,011	1,025
Pension, other post-retirement and other post-employment liabilities	745	682
Intercompany payables	833	2,178
Other non-current liabilities	836	867
TOTAL LIABILITIES	21,238	24,100
TOTAL AON SHAREHOLDERS’ EQUITY	18,282	14,247
Noncontrolling interests	60	50
TOTAL EQUITY	18,342	14,297
TOTAL LIABILITIES AND EQUITY	39,580	38,397
Consolidating Adjustments [Member]
CURRENT ASSETS :
Cash and cash equivalents	(3,714)	(1,016)	(39)	(21)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(12,425)	(9,141)
Other current assets	(33)	(19)
Total Current Assets	(16,172)	(10,176)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	0	0
Non-current deferred tax assets	(698)	(605)
Intercompany receivables	(8,110)	(11,545)
Other non-current assets	(92)	(99)
Investment in subsidiary	(20,162)	(16,301)
TOTAL ASSETS	(45,234)	(38,726)
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	0	(4)
Accounts payable and accrued liabilities	(3,714)	(1,016)
Intercompany payables	(12,425)	(9,141)
Other current liabilities	(33)	(19)
Total Current Liabilities	(16,172)	(10,180)
Long-term debt	0	4
Non-current deferred tax liabilities	(698)	(605)
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	(8,110)	(11,545)
Other non-current liabilities	(92)	(99)
TOTAL LIABILITIES	(25,072)	(22,425)
TOTAL AON SHAREHOLDERS’ EQUITY	(20,162)	(16,301)
Noncontrolling interests	0	0
TOTAL EQUITY	(20,162)	(16,301)
TOTAL LIABILITIES AND EQUITY	$ (45,234)	$ (38,726)